Some engines mounted in vehicles such as motorcycles or all terrain vehicles (ATVs) are integral with transmissions. In such engines, an oil pump is driven in synchronization with an engine speed of the engine to suction up oil from an interior of an oil pan mounted at a bottom portion of the engine or from an oil tank externally mounted to the engine, thus delivering the oil to the transmission and engine components.
To be specific, the oil outflows from the oil pump and is delivered, through an oil passage formed in an interior of the engine, to the transmission to lubricate and cool the transmission, and to a crankshaft and a camshaft to lubricate and cool these shafts. In recent years, some engines have been equipped with a hydraulically-powered variable valve timing system configured to change a rotational phase of a camshaft or a hydraulically-powered tensioner lifter. In such engines, the oil is fed to the hydraulically-powered system or component to drive them (see, e.g., Japanese Laid-Open Patent Application Publication No. Hei. 7-127661).
The engine typically runs in a wide speed range from a low engine speed to a high engine speed. In particular, the engine mounted in the motorcycle runs in a very wide speed range. Since the oil pump is driven in synchronization with the engine speed of the engine, oil pressure tends to become high when the engine is running at a high engine speed, and tends to become low when the engine is running at a low engine speed.
A relatively high oil pressure is sometimes required to drive some systems even when the engine is running at a low engine speed. However, it may sometimes be difficult to obtain such a high oil pressure with the engine running at a low engine speed. In order to solve this, a volume of the oil pump may be increased, which undesirably causes a substantial weight increase and a large friction loss in the engine.